1. Field of the Invention
The present invention relates to a sheet detecting device and an image forming apparatus.
2. Description of Related Art
The conventional image forming apparatus such as a copying machine, a facsimile machine and a printer for dealing with sheets are generally constructed in structure provided with a sheet transporting device for feeding (or transporting) each sheet to a predetermined position in synchronism with a feed signal from the main body of the apparatus.
In the sheet transporting device of this type, it is necessary to separate and feed sheets one by one from a stack of sheets stacked on a tray or in a cassette and transport each sheet to a predetermined position at predetermined timing synchronized with formation of an image, with a high degree of accuracy. For this reason, a sheet detecting device is disposed on the sheet transporting device to detect the position of the leading edge or the trailing edge of each sheet.
The sheet detecting devices are generally classified into a contact sensor type and a non-contact sensor type.
The contact sensors are detecting devices frequently used heretofore. For example, there are the known contact sensors of structure in which an inclinable lever is arranged to project in a sheet transport path and in which, with a transporting sheet coming into contact with one end of the lever, an inclination of the lever is detected by a transmissive photosensor provided at the other end.
As the recent increase in operation speed of printers raised the sheet transportation speed, the conventional contact sensors came to encounter a problem of damaging the leading edge of the sheet, however. There are thus increasing tendencies to equip the high-speed machines with an optical sensor for optically and directly reading the presence/absence of a sheet in a non-contact manner.
The optical sensor is also used for the purpose of discriminating a type of each sheet. For example, in the case of a color printer being configured to form a color image through multi-layer transfer steps of toner materials of multiple colors, in order to effect print on a light transmissive sheet for OHP, it is necessary to perform a control of switching a fixing speed to a lower speed enough to increase optical transmittance by sufficiently fusing the toner, and the optical sensor is thus arranged to detect whether the sheet is a light transmissive sheet such as an OHP sheet.
A conventional optical sheet detecting device used in the image forming apparatus will be described below with reference to FIG. 15.
The sheet transporting device of FIG. 15 is provided with a stepping motor (not shown) which drives a sheet feed load, described hereinafter, at a predetermined speed in accordance with a command from a control unit (not shown); a semicircular roller 201 which separates and feeds a sheet from a stack of sheets in a cassette 214 on the basis of a control of releasing a latch by a solenoid (not shown) and mechanically rotating the roller through one revolution; transporting rollers 202 disposed downstream of the semicircular roller 201; registration rollers 204 which are switchable between a halt and rotation by an electromagnetic clutch; a sheet presence/absence detecting sensor 203 of the contact type as an ante-registration sensor disposed immediately before the registration rollers 204; and a leading edge sensor 205 as an optical sheet presence/absence detecting means disposed downstream of the registration rollers.
The main body section as an image forming means of the image forming apparatus is provided with a photosensitive drum 211 as an image bearing member; a scanner 210 which forms an electrostatic latent image on the photosensitive drum 211; a developing device 209 which develops the electrostatic latent image with toner materials of respective colors of C, M, Y, and K; a transfer drum 207 which rotates in a state in which a sheet transported by the sheet transporting device is wound around and attached onto the transfer drum 207, and which transfers toner images of the respective colors formed on the photosensitive drum 211, onto the sheet; a stripping claw 212 which strips the sheet with the toner images transferred thereon, from the transfer drum 207; and a fixing device 213 which thermally fixes the transferred toner images on the sheet. A gripper 208 for gripping the leading edge of the sheet is provided on the transfer drum 207 and a gripper position sensor 206, which detects arrival of the gripper 208 at a position equivalent to the sheet feed position of the leading edge sensor 205, is provided in the vicinity of the transfer drum 207.
A configuration of the control unit, which controls the hardware structure as described above, will be described. When a print signal is issued, the control unit rotates the semicircular roller 201 through one revolution to feed a sheet at a predetermined speed, and also rotates the transfer drum 207.
The sheet transported by the transporting rollers 202 comes into abutment against a nip between the registration rollers 204 kept in a halt state to form a loop of a certain size, thereby implementing skew-feed correction. The registration rollers 204 then start to be rotated at a certain time after detection of the leading edge of the sheet at the ante-registration sensor 203 to lead the skew-corrected sheet in. When the leading edge sensor 205 detects the leading edge of the sheet thereafter, the registration rollers 204 are again brought into a halt state to stand by.
When the gripper sensor 206 detects arrival of the gripper on the transfer drum 207, the control unit restarts the stepping motor and controls the registration rollers 204 so that the sheet is transported at a feed speed relatively faster than the speed of the transfer drum 207 for a certain period of time and thereafter the feed speed is switched back to the same speed as the speed of the transfer drum 207.
This makes it feasible to perform such synchronous control as to close the gripper 208 while the sheet butts by a predetermined amount against the gripper 208 opening approximately 30xc2x0 relative to the surface of the transfer drum 207, and always feed the sheet stably to the gripper position as a leading edge position during the transferring operation.
The following will describe a control operation performed in feeding an OHP sheet by the sheet feed control and the optical sheet detecting device.
When an OHP sheet is fed up to the leading edge sensor 205 through the sheet feed control, the leading edge sensor 205 detects a light shield portion preliminarily printed in the width of 5 mm downstream from the leading edge on the sheet, whereupon the stepping motor is halted to stand by. When the gripper sensor 206 detects the gripper, the OHP sheet is refed. Thereafter, the leading edge sensor 205 detects a transmissive portion spaced by 20 mm and subsequent distances away from and downstream of the leading edge of the OHP sheet (or detects transmission of light) to make a judgment as an OHP sheet. Then toner images are transferred, and thereafter the control unit performs such control as to decrease the driving speed of the fixing device 213 to one third of the normal speed at the time of stripping and discharging the sheet.
A configuration of the leading edge sensor 205 will be described below referring to FIGS. 16A and 16B.
In FIGS. 16A and 16B the leading edge sensor 205 is a transmissive photosensor in which a reflecting member 126 is disposed on one side of the sheet transport path 121 and a light emitting and receiving unit 120 including a light emitting element 122 and a light receiving element 123 is disposed on the other side.
When no sheet S is present as shown in FIG. 16A, light L emitted from the light emitting element 122 travels through a slit 124 provided in a light shield cover to be reflected by the reflecting member 126, and the reflected light again travels through a slit 125 provided in the light shield cover to reach the light receiving element 123. When a sheet S is present on the other hand as shown in FIG. 16B, the light L emitted from the light emitting element 122 is shut off by the sheet S so as not to reach the light receiving element 123.
Namely, in the case of the sheet such as paper or the like, the absence of the sheet is determined with detection of light at the light receiving element 123, while the presence of the sheet is determined without detection of light. In the case of the transmissive sheet such as the OHP sheet or the like, whether the sheet is a transmissive sheet is determined based on the operation in which the light is once shut off by the light shield portion printed on the sheet and the light receiving element 123 detects the light after transportation by the predetermined amount, as described above.
The transmissive photosensor of the non-contact type as described is required to increase the S/N ratio between reflected light (signal) back from the reflecting member and reflected light (noise) back from the sheet surface, thereby raising the detection accuracy of the sheet. The slits 124, 125 are provided for the purpose of restricting the widths of the irradiated light and reflected light to restrain the reflected light back from the sheet surface from entering the light receiving element 123, thereby decreasing the noise.
However, the problem as described below was encountered in the related art case as described above.
Because of the configuration wherein the reflecting member 126 and the light emitting and receiving unit 120 are disposed on the both sides of the sheet transport path 121, a relative positional deviation is apt to occur between the two members in installation of the members. With occurrence of the positional deviation, the quantity of reflected light from the reflecting member 126 will be greatly affected.
For example, where a parallel positional deviation occurs between the reflecting member 126 and the light emitting and receiving unit 120, as shown in FIG. 17A, the spacing is expanded between the optical path of the irradiated light from the light emitting element 122 and the optical path of the reflected light back from the reflecting member 126 (the spacing is narrowed in the case of the deviation opposite to that in the same drawing), so as to cause a deviation between the optical path of the reflected light and the position of the slit 125 of the light receiving element 123, thereby significantly decreasing the quantity of reflected light detected by the light receiving element 123. When the reflecting member 126 and the light emitting and receiving unit 120 are installed with some rotational deviation, as shown in FIG. 17B, a problem similar to the above problem also occurs because of change in the spacing between the optical paths.
Since there is little change in the quantity of the reflected light back from the sheet surface in these cases on the other hand, the S/N ratio is lowered as a result to increase the risk of causing a detection error of the sheet.
In order to solve the above problem, it is conceivable that some margin is given to the slit widths so as to make allowance for some positional deviation. However, increase in the widths of the slits 124xe2x80x2, 125xe2x80x2, as shown in FIG. 18, increases the quantity of the reflected light back from the sheet surface in turn, also resulting in decrease of the S/N ratio. In addition, since the increase of the slit widths results in requiring a considerable time for the sheet S to cover the slits, variation occurs in the timing of detecting the presence of the sheet, posing another problem of degradation of the position detection accuracy of the sheet S.
The present invention has been accomplished in order to solve the above-stated problems in the related art, and an object of the invention is to provide a sheet detecting device that permits stable detection with the S/N ratio being maintained high even with the relative positional deviation between the reflecting member and the light emitting and receiving unit and that permits improvement in the position detection accuracy in the transport direction of the sheet, and to provide an image forming apparatus including the sheet detecting device.
In order to achieve the above object, a sheet detecting device according to the present invention is a sheet detecting device comprising a light emitting and receiving unit having a light emitting element for emitting detection light and a light receiving element for receiving the detection light, and a reflecting member for reflecting the detection light emitted from the light emitting element and make the detection light incident to the light receiving element, in which the light emitting and receiving unit and the reflecting member are placed with a sheet transport path between them, thereby a sheet is detected on the basis of interruption of the detection light by the sheet transported in the sheet transport path, the sheet detecting device comprising an emission slit which restricts (or stops down) the detection light emitted from the light emitting element and which is arranged so as to be longitudinal along a sheet transport direction; and a reception slit which restricts (or stops down) the detection light incident to the light receiving element and which is arranged so as to be longitudinal along a direction perpendicular to the sheet transport direction.
In a preferred configuration, the light emitting element and the light receiving element are placed so that center axes of respective optical paths thereof are approximately parallel to each other, and the reflecting member reflects the detection light approximately normally incident thereto from the light emitting element, approximately in parallel with the incident light to make the detection light incident approximately normally to the light receiving element.
In this configuration, the reflecting member is preferably constructed of an optical prism.
In another preferred configuration, a longitudinal width of the reception slit is set approximately two or more times greater than a transverse width of the emission slit.
In another preferred configuration, an area of the emission slit is set greater than an area of the reception slit.
An image forming apparatus according to the present invention comprises the sheet detecting device, and image forming means which forms an image on a sheet through control of the sheet by the sheet detecting device.
Since the present invention is based on the configuration wherein the emission slit for restricting the detection light emitted from the light emitting element is arranged so as to be longitudinal along the sheet transport direction and wherein the reception slit for restricting the detection light incident to the light receiving element is arranged so as to be longitudinal along the direction perpendicular to the sheet transport direction, it permits the stable detection with the S/N ratio being maintained high even with the relative positional deviation between the reflecting member and the light emitting and receiving unit and also permits the improvement in the position detection accuracy in the transport direction of the sheet.